Computer standby problem

I have read a lot of posts about computers not going into standby but my problem is a bit different. My computer always goes into standby automatically
after it has been powered up and the preset time has expired the FIRST time.If I restart it ,use it then leave it to shut down it doesn’t happen. It goes into standby automatically only the first time after shutdown. My wife’s computer on the same home network goes into standby every time .I am using XP Pro she has the Home version.
I would appreciate any help